1. Field of the Invention
The invention relates to charge injection devices optimized for IR sensing and more particularly to an improved readout circuit for a linear IR sensing array.
2. Prior Art
IR sensing CID arrays are well known, the sensor substrate material often being Indium Antimonide (InSb) or mercury cadmium telluride (HgCdTe). These materials are compound semiconductors which are doped to achieve a desired impurity level.
When exposed to IR, photon collisions create hole-electron pairs in the substrate. In the usual construction, an electrode is applied to the under surface of the substrate, and an oxide layer is applied to the upper surface of the substrate, followed by a transparent electrode identified with a particular sense element of the array on the upper surface of the oxide. The sensor element, with its insulated electrode, when suitably reversely biased, stores IR induced charges (the holes) in a potential well. If the reverse bias is removed, the charges are injected into the substrate emptying the well. Readings reflecting the IR induced charge on the sensor may be taken before and after injection if one wishes to measure the IR intensity. In practical devices, where there are a large number of elements, as in a linear IR array, there is a need to take a large number of readings in a relatively short time. In practice, each reading of a sensor element is a double sample, and the readout process requires that the charge stored in the potential well be "injected" into the substrate. The injection, however, is not instantaneous, and as the injection step is shortened below several microseconds, a substantial percentage of uninjected charge occurs, and the sensitivity of the IR sensor is reduced. For instance, if the injection time of the sensor can be as long as 10 microseconds, the lag described is negligible. On the other hand, if the injection time is in smaller than 10 nanoseconds only a very small percentage of charge is injected.
In CID IR sensor arrays of practical interest, the readout interval allocated to each sensor element has become so short, (e.g. 1.6 microsecond) that the injection time, which is a part of the readout interval, is so small that the device lag presents a significant reduction in performance.